This invention relates to xerography and particularly concerns a means and method for cleaning the corotron electrode wires used in xerographic copying machines.
In the process of xerography, as disclosed for example in Carlson U.S. Pat. No. 2,297,691, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually be conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely-divided material such as an electroscopic powder that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.
In automatic equipment employing the principles of xerography a plate in the form of a cylindrical drum is continuously rotated through a cycle of sequential operations including charging, exposure, developing and transfer. During the cycle a copy is reproduced onto a support surface and after transfer, the plate is cleaned before re-use. It is usual to charge the xerographic plate with corona of positive polarity on the order of 800-1100 volts by means of corona generating device having an electrode which, when supplied with potential above the corona threshold, produces an emission of corona ions that deposit uniformly onto the plate surface. Typical of the corona generating device employed heretofore are those described in U.S. Pat. No. 2,777,957 and U.S. Pat. No. 2,836,725 which show an electrode wire or wires supported relatively close to the surface to be charged. A grounded metallic shield generally surrounds the electrode except for an opening through which charge is emitted and is adapted to attract surplus emission emanating therefrom.
Inherent in xerographic apparatus of the type described above is the continuous presence of dust generated by the operations and generally comprising particulate quantities of stray electroscopic powder. With prolonged continuous operation, it has been found that the dust accumulates on and about the interior of the corona generator to such an extent that the charging efficiency thereof decreases substantially as the density of dust accumulation increases. This has required frequent cleaning and maintenance. For example, in order to maintain the entire apparatus operating effectively it has been necessary heretofore to completely clean the corona generator after approximately every 20,000 reproductions made. With millions of copies being generated yearly, the cost of servicing has represented a significant economic overhead. At the same time, in order to ensure uninterrupted operation within a plausible programmed maintenance schedule, it has been found necessary to operate these prior corona generating devices at a potential substantially above the threshold potential, being a much higher potential than is otherwise required for corona emission. Still further, in addition to the problem associated with cleanliness it has long been known that the consumption of corona collected by the surrounding shield has been far in excess of that applied onto a recipient surface to be charged. It has been established that with some corona generators the shield or other surrounding elements consume as much as 84% of the total charging current while only the remaining 16% is effective in applying a charge in the manner intended onto a receiving surface such as a xerographic plate.
Although it has been known for some time as shown, for example, by U.S. Pat. No. 2,547,573 and U.S. Pat. No. 3.094,437 that corona generating precipitator electrode wires may be cleaned by intermittently displacing the wire by rapping it or vibrating it, the concept of cleaning a corotron electrode wire of an electrostatic copying machine by causing it to vibrate was not considered apparently because of different conditions under which it operated and the different problems presented, i.e., the possibility of arcing which occurs in copying machines. These differences were recognized by the prior art as shown in U.K. Pat. No. 1,116,687. In that patent, as well as U.K. Pat. No. 1,331,339 the suggested solution is a grounded shield and reduction or prevention of vibration of the wire. Other prior art as shown for example in U.S. Pat. Nos. 2,811,135; 3,842,273; and 3,875,407 suggests mechanical flexing and scraping along the length of the wire while others, for example, disclose replacing the dirty wire (U.S. Pat. No. 3,499,143), vaporising the dirt with periodic high temperature surges (U.S. Pat. No. 3,496,352) and blowing clean air over the wires, (U.S. Pat. No. 3,324,291).
It is a discovery of this invention that periodic controlled vibration of the corotron wire in a copying machine indeed does clean the wire to produce detectable increases in copy quality and in fact does not have the disadvantages such as arcing heretofore associated therewith.